The present invention is concerned with an improved apparatus and method for photometric analysis of fluid samples and the like. More particularly, the present invention is concerned with a sample cell and method for photometric analysis of fluid flows.
As is well known, photometric analysis provides information on the chemical composition of a sample fluid. In many different industrial processes, such as petroleum refineries, pharmaceutical plants, chemical plants, wastewater plants and the like, measuring the chemical composition of a fluid stream is beneficial for the purpose of characterizing the properties of that stream for quality assurance or process control. Photometric analysis can also be used on power plants, fuel cells, or propulsion engines to characterize the fuel and lubrication oil for the purpose of detecting contaminants, wear debris, or thermal degradation.
In a flowing system, such as a chemical process, wastewater treatment plant, combustion engine or the like, the fluid is exposed for measurement in a flow cell or sample cell having windows to allow the transmittance of light from a light source to a detector. The present invention, generally speaking, is directed towards such a flow cell in which fiber optic ports allow attachment of flexible fiber optic cables. In such a flow cell, an illumination cable carries light from the light source such as a laser, light emitting diode, or a broadband lamp to the flow cell, and a read cable carries light from the flow cell to the a detector such as a photodetector or spectrometer. Attenuation of the transmitted light is caused by absorption or scattering of light by the fluid or by particles suspended in the fluid, as the fluid passes through the flow cell which has an inlet and outlet connection for the flowing fluid.
Probes are also used for photometric analysis of fluids, either in a tank vessel or a pipe. Distinct from the flow cell, a probe is inserted into a measurement port, usually a threaded bung, on the vessel or pipe. Probes can measure transmission in a so-called transflectance probe or transmission dip probe, or measure attenuated total reflectance, or measure Raman scattering. Transflectance probes have a cylindrical probe body with an open cavity in the tip of the body that allows fluid to pass through the measurement location. The light traverses through the cavity and is reflected off an angled mirror onto an optic element, such as a lens or read fiber. In addition to the transmitted light that is attenuated due to absorbance, the read-optic also receives backscattered light from the fluid and any suspended particles in the fluid. This backscattered light causes measurement noise, decreases the signal-to-noise ratio, and leads to an ill-defined optical path length. Transmission flow cells do not suffer from backscattering. As a result, transmission flow cells have better optical signal performance than transflectance probes. However, transflectance probes have the advantage that they package nicely because the read- and illumination-fiber can be bundled together in birfurcated fiber optic cable.
Flow cells commonly have a cross configuration where the optical ports are on opposite sides and the fluid ports are also on opposite sides, thereby forming a cross. A disadvantage of cross flow cells is that the fiber optic cables are on opposite sides of the flow cell. Because the fiber optic cables have a large minimum bend radius to avoid breaking the fiber, the cables must form excessively large loops before being bundled together and running to the photodetector and light source. Often times the minimum bend radius is 4 to 8 inches, which commonly is substantially larger than the flow cell to which the cables are connected. The large looping fiber optics are a significant limitation for measurement applications where packaging is tight and space is constrained such as in a vehicle or aircraft. The fiber optic cables are fragile, and the end terminations of the cable are often a failure point. In addition, the loops in the cables provide a risk of snagging and thereby damaging the cable. Other known approaches use a folded optical configuration similar to the transflectance probe that positions the optical connections on the same face of the flow cell; however, these flow cells also suffer from backscattering like the transflectance probes.
The present invention deals with a flow cell and method for transmission measurements in fluid that address the foregoing problems and shortcomings of known approaches. Our flow cell includes a folded optical path that provides for co-located, collinear fiber optic cables. The light passes through the sample, is reflected off a pair of turning mirrors and returns through the fluid for a second pass. As such the read and illumination cables can be bundled together, greatly easing the packaging constraints inherent with fiber optic cables. Unlike known folded optical path flow cells, the present invention also includes a center divider that separates the fluid passages on the first pass of the light through the fluid and the second pass of the light through the fluid. This center divider blocks any undesired backscattered light and does not allow that light to reach the read optic. Our invention provides the ease of fiber optic packaging of a transflectance probe and the optical performance of a cross flow cell.
An object of our invention is to provide an improved transmission flow cell and method for photometric analysis of fluid flows with a minimal backscattered stray light and a well defined optical path length.
A further object of our invention is to provide a flow cell with a folded optical path that allows for compact, rugged packaging of the fiber optic cables.
The present invention improves on the known variations of fluid flow cells and fluid probes by offering co-located, collinear fiber optic ports and a center divider that eliminates backscattered light from reaching the detector.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings and non-limiting examples herein.